Emulsion copolymers prepared from vinyl acetate, ethylene and a crosslinkable comonomer are widely used as binders in the nonwovens industry. Unfortunately, these binders often have unacceptably short shelf-life and when applied to a nonwoven substrate exhibit unacceptable loss in strength in the presence of water and organic solvents. Such shortcomings limit the use of these formulations in many commercial and industrial applications. Consequently, the nonwovens industry has been searching for a binder prepared by a one-pot preparation which can overcome these limitations.
Numerous formulations have been developed in an effort to overcome problems associated with vinyl acetate/ethylene (VAE) emulsions. The most successful of the many chemistries employed is aminoplast technology wherein N-methylol acrylamide (NMA) or urea-formaldehyde (U/F) condensates are used as crosslinking agents. While these compounds are low in cost, compatible with aqueous emulsions and are rapidly cured under acid catalysis, their use is limited because they exhibit unacceptable resistance to solvents. Additionally, the cured products tend to exhibit inadequate adhesion to certain substrates, including metal, glass and synthetics such as mylar.
Most attempts at solving problems associated with diminished emulsion shelf life and low resistance of the bonded substrates from attack by organic solvents have been unsuccessful for a variety of reasons. For example, the inclusion of additional crosslinkable comonomer into the polymer premix improves binder performance until the comonomer content reaches a level wherein the crosslinkable monomer undergoes block polymerization. Use of melamine/formaldehyde resins fails because they are either unreactive under standard process operating temperatures or are too reactive with the functional latex thereby resulting in an emulsion having limited shelf life. Moreover, melamine/formaldehyde resins are typically unable to react to form oligomers which can bridge the binder to other chains of the binder or to the substrate.
U.S. Pat. No. 3,380,851 discloses a nonwoven binder comprising an interpolymer of vinyl acetate, ethylene and NMA wherein the interpolymer contains 5 to 40 wt % ethylene and 0.5 to 10 wt % NMA (based upon vinyl acetate).
U.S. Pat. No. 4,064,191 discloses a coating composition containing an alkylated glycoluril, a polymeric non-self-crosslinking compound and an acid catalyst. Suitable compositions comprise a blend of 2 to 50 wt % of the following components:
(A) a partially or fully alkylated glycoluril derivative such as tetramethylol glycoluril, a partially methylolated glycoluril, and the like; PA1 (B) less than 25 wt % of an organic solvent soluble, normally non-self-crosslinking polymeric material having as reactive groups, any one or more of carboxyl groups, alcoholic hydroxyl groups or amide groups; and PA1 (C) from about 0.05 to 5.0 wt % of an acid catalyst based on the total weight of (A) and (B), wherein the reactive groups of (B) are heat reactive with (A) and wherein the percentages by weight of (A) and (B) total 100% and are based on the total solids weight of (A) and (B).
U.S. Pat. No. 4,442,257 teaches thermosetting low temperature cure coating compositions produced by combining certain emulsion polymers containing reactive hydroxyl groups, carboxyl groups, and acrylamide groups with dimethylol dihydroxy ethylene urea whereby the hydroxy groups crosslink with the dimethylol dihydroxy ethylene urea upon curing. Especially preferred are acrylamide polymers.
U.S. Pat. No. 4,444,941 discloses an acrylamide emulsion copolymer containing at least about 1.5% of a copolymerized N-alkylol amide as well as a carboxyl-containing monomer and a hydroxyl-containing monomer. The copolymerization of about 8 wt % monomers with other ethylenic monomers provides a reactive emulsion polymer adapted to be crosslinked with a glycoluril derivative upon moderate heat curing to form a cured coating composition. The acrylamide monomer, the carboxyl monomer, and the hydroxyl monomer are copolymerized together with other ethylenically unsaturated monomers in an aqueous emulsion polymerization process to produce a reactive emulsion polymer. A glycoluril derivative is added to the emulsion to provide a thermosetting emulsion mixture adapted to cure and crosslink upon application of moderate heat.
U.S. Pat. No. 4,487,889 discloses a protective surface coating composition containing a polymeric binder, the binder comprising on a weight basis; between 5% and 60% polyol containing two or more hydroxyl groups, between 2% and 60% of a glycoluril derivative and between 10% and 80% emulsion polymer, the emulsion polymer being emulsion copolymerized ethylenic monomers. The thermosetting composition can be cured to form a coating film. Column 4, lines 39-47 states that useful glycoluril derivatives include tetramethylol glycoluril, mono- and dimethylethers of dimethylol glycoluril, the trimethylether of tetramethylol glycoluril, tetrakishexoxymethyl glycoluril and the like.
U.S. Pat. No. 4,542,180 discloses an improved coating composition which comprises a coreactive polymeric mixture of a reactive emulsion polymer produced by copolymerizing ethylene monomers in the presence of a glycoluril derivative dispersed in water to provide an in-situ blend of reaction polymer and a glycoluril derivative. Glycoluril derivatives useful for practicing the invention include tetramethylol glycoluril, mono- and dimethylethers of dimethylol glycoluril, the trimethylether of tetramethylol glycoluril, tetrakishexoxymethyl glycoluril and the like. Upon curing, the glycoluril derivative crosslinks with the reactive in-situ formed emulsion polymer. Example 2 discloses a latex catalyzed by adding a 40% solution of para-toluenesulfonic acid in isopropanol equal to 6% of the weight of tetramethylol glycoluril solids. A latex of similar composition prepared by post addition of tetramethylol glycoluril did not form an acceptable film.
U.S. Pat. No. 4,647,611 discloses a process for preparing a nonwoven binder emulsion containing a crosslinkable vinyl acetate or vinyl acetate/ethylene copolymer prepared by polymerizing in an aqueous dispersion vinyl acetate, or vinyl acetate and ethylene in the presence of a crosslinkable comonomer of the formula: EQU R--NH--(--CH.sub.2).sub.n --CH(OR.sup.1).sub.2
wherein R is a C.sub.3 -C.sub.10 olefinically unsaturated organic radical having functionality which renders the nitrogen atom electron deficient, R.sup.1 is hydrogen or C.sub.1 -C.sub.4 alkyl group and n is 3 or 4.
U.S. Pat. No. 4,774,283 discloses a copolymer emulsion for bonding nonwovens which comprises an aqueous medium having colloidally dispersed therein a copolymer consisting essentially of vinyl acetate, 1 to 20 wt % ethylene, 0.5-15 wt % (based on vinyl acetate) crosslinkable comonomer having the above-mentioned formula or N-acrylamidoglycolic acid, and 0.1 to 5 wt % of an acrylamide.